1. Field of the Invention
The present invention relates to optical waveguide gratings, particularly to radiative mode-coupled optical waveguide gratings.
2. Description of the Related Art
Optical waveguide gratings are optical fibers or planar optical waveguides having constant periodic changes in the longitudinal direction, such as periodic changes in the refractive index of the core or periodic changes in the core diameter.
In general, gratings can be divided into radiative mode-coupled types and reflective mode-coupled types. Radiative mode-coupled gratings are capable of attenuating light of specific wavelengths due to radiation from the optical waveguide by coupling modes propagating in the core with modes propagating in the cladding. Reflective mode-coupled gratings reflect light of specific wavelengths by coupling modes propagating through the core in a positive direction and modes propagating through the core in the opposite direction (negative direction).
Mode-coupling is made possible by perturbations which occur in the core. Generally, when gratings are formed in optical fibers, these perturbations are often achieved by means of periodic changes in the core refractive index.
The main structural difference between radiative mode-coupled gratings and reflective mode-coupled gratings is in the periods of the periodic changes (hereinafter referred to as the grating pitch). For example, in the case of optical fiber gratings formed by making periodic changes in the core refractive index of optical fibers, radiative gratings are obtained by making the grating pitch approximately several hundred microns, and reflective gratings are obtained by making the grating pitch approximately 1 micron.
Radiative mode-coupled gratings have wavelength-transmission loss properties (transmission spectra) as shown in FIG. 7, wherein the transmission loss of light in a specific wavelength band is selectively increased. The width of the wavelength band with an increased transmission loss is referred to as the rejection bandwidth, the central wavelength thereof is referred to as the central wavelength of the rejection band, and the magnitude of the change in transmission loss is referred to as the rejection.
As a conventional method for producing optical waveguide gratings, there is a method for making periodic refractive index changes in the core by taking advantage of the properties of silica glass doped with germanium, of which the refractive index will increase when exposed to strong UV radiation, depending on the amount of exposure.
For example, when producing a radiative mode-coupled optical fiber grating, either an optical fiber with a germanium-doped core and a silica cladding, or an optical fiber with a germanium-doped core and a fluorine-doped cladding is used. This optical fiber is hydrogenated in a hydrogen-pressurized container (approximately 100 atm), and then either exposed to UV radiation at constant periods along the longitudinal direction of the optical fiber using a photomask, or exposed to UV radiation at regularly spaced intervals along the longitudinal direction of the optical fiber.
However, conventional radiative mode-coupled optical fiber gratings made from optical fibers with germanium-doped cores and silica claddings or optical fibers with germanium-doped cores and fluorine-doped claddings have the undesirable property that the central wavelength of the rejection band has a high temperature dependence.
Specifically, this type of optical fiber grating has a temperature characteristic of approximately 0.05 nm/.degree.C., meaning that as the temperature rises (or drops) by 10.degree. C., the central wavelength of the rejection band will shift to longer wavelengths (or shorter wavelengths) by approximately 0.5 nm. Therefore, they are not dependable in terms of their stability and reliability as optical components.
On the other hand, the grating properties of optical waveguide gratings are known to change with the parameters of the gratings, i.e. the amount of change in the core refractive index, the grating pitch, the grating shape (profile of the core refractive index), the grating length in the longitudinal direction of the optical fiber, and the effective refractive index.
The following Table 1 summarizes the influence that each parameter of a grating has on the grating properties. In the table, x indicates no influence, O indicates some influence, and .DELTA. indicates a small influence. Additionally, the arrows .Arrow-up bold. (.arrow-down dbl.) indicate whether the value of the grating property will increase decrease) in response to an increase in the parameter value.
TABLE 1 ______________________________________ Central Rejection PARAMETER Wavelength Rejection Bandwidth ______________________________________ Change in Refractive Index .smallcircle..uparw. .smallcircle..uparw. x Grating Pitch .smallcircle..uparw. .DELTA. x Grating Shape .smallcircle. .smallcircle. x Grating Length x .smallcircle..uparw. .smallcircle..dwnarw. Effective Refractive Index .smallcircle..uparw. x x ______________________________________
Radiative mode-coupled optical fiber gratings can be used in the field of optical communications, and are especially suitable for use in order to reduce the wavelength dependence of the gain in erbium-doped optical fiber amplifiers in optical communication systems which perform wavelength-division-multiplexed transmissions. In this case, the radiative mode-coupled optical fiber grating should preferably be designed so that the rejection band is the same as the wavelength region used for transmission.
For example, FIG. 8 shows the wavelength dependence of the gain of a erbium-doped optical fiber amplifier, and this optical fiber amplifier can be used in an optical communication system for performing wavelength-division multiplexed transmissions between wavelength A and wavelength B.
The optical fiber grating used in this optical communication system should be designed such as to have a rejection band which overlaps the wavelength region between wavelength A and wavelength B, and such that the wavelength-transmission loss properties in this wavelength region form a curve similar to the wavelength dependence of the gain in the same wavelength region (the curve in the graph). This type of design efficiently equalizes the gain in this wavelength region.
Conventionally, the only known method for controlling the rejection band width of a radiative mode-coupled grating is to adjust the grating length as indicated in Table 1 above.
However, the wavelength region used in wavelength-division multiplexed transmission is usually determined by the gain band of the erbium-doped optical fiber amplifier, and this is the wavelength region between wavelength A and wavelength B in FIG. 8. While the bandwidth is approximately 15-20 nm, if a radiative mode-coupled grating having a relatively narrow rejection bandwidth corresponding thereto is to be made, the grating length becomes extremely long.
For example, optical fibers identical to the optical fibers used for communications are conventionally used to make optical fiber gratings, but in order to make the rejection bandwidth less than 15 nm with this type of optical fiber, the grating length must be at least 50 mm.
If the grating length of an optical fiber grating is too long, it becomes insuitable for compact optical components, and is not capable of being contained in existing repeaters.
Additionally, while optical fiber gratings are normally used with both sides of the cladding portion affixed to a substrate or the like by means of an adhesive, the resonance frequency of the grating portion is reduced if the grating length is increased, so that there is a risk of the grating portion resonating during vibration testing or during the installation of repeaters.
Palnar optical waveguides can similarly be formed into radiative mode-coupled gratings or reflective mode-coupled gratings by means of perturbations in the core.
These perturbations can be achieved relatively easily by means of periodic changes in the core diameter (core width) of the waveguide in the case of radiative mode-coupled gratings, and can be achieved by means of changes in the core refractive index of the waveguide in the case of reflective mode-coupled gratings due to the shortness of the grating pitch.
However, radiative mode-coupled gratings formed in palnar optical waveguides have a problem in that their rejections cannot be made sufficiently large in comparison to radiative mode-coupled gratings in optical fibers.
That is, while the rejection changes periodically if the grating length is increased in a radiative mode-coupled grating, in the case of an optical fiber grating, the period of the changes in the rejection is comparatively long and the amount of change is large, so that the rejection can be monotonically increased if the grating length in increased within the normally used range.
In contrast, in radiative mode-coupled palnar optical waveguide gratings, the period for the rejection change is comparatively short and the amount of change is small, so that the rejection cannot be made greater than a certain value because the rejection will simply change periodically even if the grating length is increased.